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Trigger EventHandler of one ViewModel in another ViewModel

I have achieved desired result with MessagingCenter, but I have got an information from reading Xamarin articles that MessagingCenter is not the preferred way to trigger 30+ events. Additional to that I have to unsubscribe from MessagingCenter after action has been done. I want to have Settings page where I would have 30+ settings that have to be changed across whole application in different views. How I can inject SettingsViewModel into other ViewModels in Xamarin.Forms application?
SettingsViewModel.cs:
namespace MessagingCenterApp.ViewModels
{
public class SettingsViewModel : BaseViewModel, ISettingsViewModel
{
public ICommand ChangeCommand { get; set; }
public SettingsViewModel()
{
Title = "Settings";
this.BoxColor = Color.Red;
this.ChangeCommand = new Command(this.ChangeColor);
}
private void ChangeColor()
{
this.BoxColor = Color.FromHex(this.BoxColorS);
MessagingCenter.Send<Object, Color>(this, "boxColor", this.BoxColor);
}
private Color _boxColor;
public Color BoxColor
{
get => _boxColor;
set
{
_boxColor = value;
this.OnPropertyChanged();
}
}
private string _boxColorS;
public string BoxColorS
{
get => Preferences.Get("BoxColor", "#17805d");
set
{
Preferences.Set("BoxColor", value);
this.ChangeColor();
this.OnSettingsChanged();
this.OnPropertyChanged();
}
}
public event EventHandler<SettingsChangedEventArgs> SettingsChanged;
private void OnSettingsChanged() => this.SettingsChanged?.Invoke(this, new SettingsChangedEventArgs(this.Settings));
public Settings Settings { get; private set; }
}
}
HomeViewModel.cs:
namespace MessagingCenterApp.ViewModels
{
public class HomeViewModel : BaseViewModel
{
public HomeViewModel()
{
this.Title = "Home";
MessagingCenter.Subscribe<Object, Color>(this, "boxColor", (sender, arg) =>
{
System.Diagnostics.Debug.WriteLine("received color = " + arg);
this.BoxColor = arg;
});
this.BoxColor = Color.Red;
this.SettingsViewModel = new SettingsViewModel();
this.SettingsViewModel.SettingsChanged += OnSettingsChanged;
}
private void OnSettingsChanged(object sender, SettingsChangedEventArgs e)
{
throw new NotImplementedException();
}
private Color _boxColor;
public Color BoxColor
{
get => _boxColor;
set
{
_boxColor = value;
OnPropertyChanged();
}
}
private ISettingsViewModel SettingsViewModel { get; }
}
}
Should I somehow do all in MainViewModel? I mean:
namespace MessagingCenterApp.ViewModels
{
public class MainViewModel : BaseViewModel
{
public MainViewModel()
{
this.SettingsViewModel = new SettingsViewModel();
this.HomeViewModel = new HomeViewModel(this.SettingsViewModel);
}
public SettingsViewModel SettingsViewModel { get; set; }
public HomeViewModel HomeViewModel { get; }
}
}
Then initialized it in AppShell? I could not get this approach working.
Important! I don't want to use any MVVM framework! Only native behaviour.

mvvmcross' Messenger is alleged to be "lighter weight" than X-Form's built-in Messaging Center.
I use mvvmcross Messenger by defining some helper methods in a "BasePage". Then each page inherits from "BasePage" rather than "ContentPage".
This automatically handles "unsubscribe" of each method. And makes it easier to manage mvvmcross' "subscription tokens".
BasePage.xaml.cs:
// If not using mvvmcross, this could inherit from ContentPage instead.
public class BasePage : MvxContentPage
{
protected readonly IMvxMessenger Messenger;
public BasePage()
{
this.Messenger = Mvx.IoCProvider.Resolve<IMvxMessenger>();
}
protected override void OnAppearing()
{
base.OnAppearing();
// Examples of subscribing to messages. Your subclasses of BasePage can also do this.
this.Subscribe<MyMessage1>(OnMyMessage1);
this.SubscribeOnMainThread<MyMessage2>(OnMyMessage2);
}
protected override void OnDisappearing()
{
UnsubscribeAll();
base.OnDisappearing();
}
#region Messenger Subscriptions
protected List<MvxSubscriptionToken> _subscriptions = new List<MvxSubscriptionToken>();
/// <summary>
/// Create subscription and add to "_subscriptions".
/// Call this from subclass' OnAppearing, once per subscription.
/// Automatically unsubscribed in OnDisappearing.
/// </summary>
/// <param name="token"></param>
/// <param name="msgType"></param>
protected void Subscribe<T>(Action<T> onMessage) where T : MvxMessage
{
var token = this.Messenger.Subscribe<T>(onMessage);
// Hold token to avoid GC of the subscription.
_subscriptions.Add(token);
}
protected void SubscribeOnMainThread<T>(Action<T> onMessage) where T : MvxMessage
{
var token = this.Messenger.SubscribeOnMainThread<T>(onMessage);
// Hold token to avoid GC of the subscription.
_subscriptions.Add(token);
}
/// <summary>
/// OnDisappearing calls this.
/// </summary>
private void UnsubscribeAll()
{
if (_subscriptions.Count > 0)
{
foreach (MvxSubscriptionToken token in _subscriptions)
{
// Per "https://www.mvvmcross.com/documentation/plugins/messenger", this is sufficient to Unsubscribe:
// "Subscriptions can be cancelled at any time using the Unsubscribe method on the IMvxMessenger or by calling Dispose() on the subscription token."
token.Dispose();
}
_subscriptions.Clear();
}
}
#endregion
}
For view models, class would be "BaseViewModel", that your view models inherit from. Contents similar to above, but different method names for Appearing/Disappearing.
BaseViewModel.cs:
public class BaseViewModel : MvxViewModel
{
...
// mvvmcross' MvxViewModel provides these.
protected override void ViewAppearing()
{
...
}
protected override void ViewDisappearing()
{
...
}
... Messenger Subscriptions methods ...
}

C# trigger event

I use C# and I want to trigger an event from within a class :
So if the Price property of a class was changed then an event onPriceChanged (outside the class) should be fired.
However, I get an error:
The name 'onPriceChanged' does not exist in the current context
How could I fix this?
(I guess that I could pass the eventhandler to the class via constructor...but if possible I would prefer not to pass the eventhandler to the class)
Here is my code :
using System;
public delegate void delEventHandler();
class clsItem
{
//private static event delEventHandler _show;
private delEventHandler _show;
private int _price;
public clsItem() //Konstruktor
{
_show += new delEventHandler(Program.onPriceChanged); // error here : The name 'onPriceChanged' does not exist in the current context
}
public int Price
{
set
{
_price = value;
_show.Invoke(); //trigger Event when Price was changed
}
}
}
class Program
{
static void Main()
{
clsItem myItem = new clsItem();
myItem.Price = 123; //this should trigger Event "onPriceChanged"
}
//EventHandler
public static void onPriceChanged()
{
Console.WriteLine("Price was changed");
}
}

You're doing this the wrong way round - you're trying to attach the event handler from the class, and clearly that cannot have access to the Program.onPriceChanged method!
You should expose your event, and attach the event handler from the client code (Program).
class clsItem
{
//private static event delEventHandler _show;
private delEventHandler _show;
private int _price;
public clsItem() //Konstruktor
{
}
public event delEventHandler Show
{
add { _show += value; }
remove { _show -= value; }
}
public int Price
{
set
{
_price = value;
_show?.Invoke(); //trigger Event when Price was changed
}
}
}
And:
clsItem myItem = new clsItem();
myItem.Show += onPriceChanged;
myItem.Price = 123; //this now does trigger Event "onPriceChanged"
Live example: http://rextester.com/WMCQQ40264

The way you're dealing with events is not a good practice. the reason why we use Events is to decouple the objects we create from the methods they need to call.
For example if you want to create another object of the same type(clsItem) and get it to call another method once its price changed, you get into trouble. So I'd suggest this code rather than the current one:
using System;
public delegate void delEventHandler();
class clsItem
{
public event delEventHandler PriceChanged;
private int _price;
public clsItem() //Konstruktor
{
}
public int Price
{
set {
if(value!=_price) // Only trigger if the price is changed
{
_price = value;
if(PriceChanged!=null) // Only run if the event is handled
{
PriceChanged();
}
}
}
}
}
class Program
{
static void Main()
{
clsItem myItem = new clsItem();
myItem.PriceChanged += new delEventHandler(onPriceChanged);
myItem.Price = 123; //this should trigger Event "PriceChanged" and call the onPriceChanged method
}
//EventHandler
public static void onPriceChanged()
{
Console.WriteLine("Price was changed");
}
}

Here is the more traditional way of doing what you want:
public delegate void delEventHandler();
class clsItem
{
public event delEventHandler Show;
private int _price;
public clsItem() //Konstruktor
{
}
public int Price
{
set
{
_price = value;
Show?.Invoke(); //trigger Event when Price was changed
}
}
}
class Program
{
static void Main()
{
clsItem myItem = new clsItem();
myItem.Show += onPriceChanged;
myItem.Price = 123; //this should trigger Event "onPriceChanged"
}
//EventHandler
public static void onPriceChanged()
{
Console.WriteLine("Price was changed");
}
}
Notice that clsItem no longer knows who is subscribing to its event. All it cares about is notifying any listeners who happens to be subscribed. There is no longer a dependency between clsItem and the onPriceChanged method.

List<T> firing Event on Change

I created a Class EventList inheriting List which fires an Event each time something is Added, Inserted or Removed:
public class EventList<T> : List<T>
{
public event ListChangedEventDelegate ListChanged;
public delegate void ListChangedEventDelegate();
public new void Add(T item)
{
base.Add(item);
if (ListChanged != null
&& ListChanged.GetInvocationList().Any())
{
ListChanged();
}
}
...
}
At the Moment I use it as a Property like this:
public EventList List
{
get { return m_List; }
set
{
m_List.ListChanged -= List_ListChanged;
m_List = value;
m_List.ListChanged += List_ListChanged;
List_ListChanged();
}
}
Now my Problem is, can I somehow handle if a new Object is referred to it or prevent that, so I do not have to do the event wiring stuff in the setter?
Of course, I can change the property to "private set" but I would like to be able to use the class as variable as well.

You seldom create a new instance of a collection class in a class. Instantiate it once and clear it instead of creating a new list. (and use the ObservableCollection since it already has the INotifyCollectionChanged interface inherited)
private readonly ObservableCollection<T> list;
public ctor() {
list = new ObservableCollection<T>();
list.CollectionChanged += listChanged;
}
public ObservableCollection<T> List { get { return list; } }
public void Clear() { list.Clear(); }
private void listChanged(object sender, NotifyCollectionChangedEventArgs args) {
// list changed
}
This way you only have to hook up events once, and can "reset it" by calling the clear method instead of checking for null or equality to the former list in the set accessor for the property.
With the changes in C#6 you can assign a get property from a constructor without the backing field (the backing field is implicit)
So the code above can be simplified to
public ctor() {
List = new ObservableCollection<T>();
List.CollectionChanged += OnListChanged;
}
public ObservableCollection<T> List { get; }
public void Clear()
{
List.Clear();
}
private void OnListChanged(object sender, NotifyCollectionChangedEventArgs args)
{
// react to list changed
}

ObservableCollection is a List with a CollectionChanged event
ObservableCollection.CollectionChanged Event
For how to wire up the event handler see answer from Patrick. +1
Not sure what you are looking for but I use this for a collection with one event that fires on add, remove, and change.
public class ObservableCollection<T>: INotifyPropertyChanged
{
private BindingList<T> ts = new BindingList<T>();
public event PropertyChangedEventHandler PropertyChanged;
// This method is called by the Set accessor of each property.
// The CallerMemberName attribute that is applied to the optional propertyName
// parameter causes the property name of the caller to be substituted as an argument.
private void NotifyPropertyChanged( String propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public BindingList<T> Ts
{
get { return ts; }
set
{
if (value != ts)
{
Ts = value;
if (Ts != null)
{
ts.ListChanged += delegate(object sender, ListChangedEventArgs args)
{
OnListChanged(this);
};
}
NotifyPropertyChanged("Ts");
}
}
}
private static void OnListChanged(ObservableCollection<T> vm)
{
// this will fire on add, remove, and change
// if want to prevent an insert this in not the right spot for that
// the OPs use of word prevent is not clear
// -1 don't be a hater
vm.NotifyPropertyChanged("Ts");
}
public ObservableCollection()
{
ts.ListChanged += delegate(object sender, ListChangedEventArgs args)
{
OnListChanged(this);
};
}
}

If you do not want to or can not convert to an Observable Collection, try this:
public class EventList<T> : IList<T> /* NOTE: Changed your List<T> to IList<T> */
{
private List<T> list; // initialize this in your constructor.
public event ListChangedEventDelegate ListChanged;
public delegate void ListChangedEventDelegate();
private void notify()
{
if (ListChanged != null
&& ListChanged.GetInvocationList().Any())
{
ListChanged();
}
}
public new void Add(T item)
{
list.Add(item);
notify();
}
public List<T> Items {
get { return list; }
set {
list = value;
notify();
}
}
...
}
Now, for your property, you should be able to reduce your code to this:
public EventList List
{
get { return m_List.Items; }
set
{
//m_List.ListChanged -= List_ListChanged;
m_List.Items = value;
//m_List.ListChanged += List_ListChanged;
//List_ListChanged();
}
}
Why? Setting anything in the EventList.Items will call your private notify() routine.

I have a Solution for when someone calls the Generic method from IList.add(object). So that you also get notified.
using System;
using System.Collections;
using System.Collections.Generic;
namespace YourNamespace
{
public class ObjectDoesNotMatchTargetBaseTypeException : Exception
{
public ObjectDoesNotMatchTargetBaseTypeException(Type targetType, object actualObject)
: base(string.Format("Expected base type ({0}) does not match actual objects type ({1}).",
targetType, actualObject.GetType()))
{
}
}
/// <summary>
/// Allows you to react, when items were added or removed to a generic List.
/// </summary>
public abstract class NoisyList<TItemType> : List<TItemType>, IList
{
#region Public Methods
/******************************************/
int IList.Add(object item)
{
CheckTargetType(item);
Add((TItemType)item);
return Count - 1;
}
void IList.Remove(object item)
{
CheckTargetType(item);
Remove((TItemType)item);
}
public new void Add(TItemType item)
{
base.Add(item);
OnItemAdded(item);
}
public new bool Remove(TItemType item)
{
var result = base.Remove(item);
OnItemRemoved(item);
return result;
}
#endregion
# region Private Methods
/******************************************/
private static void CheckTargetType(object item)
{
var targetType = typeof(TItemType);
if (item.GetType().IsSubclassOf(targetType))
throw new ObjectDoesNotMatchTargetBaseTypeException(targetType, item);
}
#endregion
#region Abstract Methods
/******************************************/
protected abstract void OnItemAdded(TItemType addedItem);
protected abstract void OnItemRemoved(TItemType removedItem);
#endregion
}
}

If an ObservableCollection is not the solution for you, you can try that:
A) Implement a custom EventArgs that will contain the new Count attribute when an event will be fired.
public class ChangeListCountEventArgs : EventArgs
{
public int NewCount
{
get;
set;
}
public ChangeListCountEventArgs(int newCount)
{
NewCount = newCount;
}
}
B) Implement a custom List that inherits from List and redefine the Count attribute and the constructors according to your needs:
public class CustomList<T> : List<T>
{
public event EventHandler<ChangeListCountEventArgs> ListCountChanged;
public new int Count
{
get
{
ListCountChanged?.Invoke(this, new ChangeListCountEventArgs(base.Count));
return base.Count;
}
}
public CustomList()
{ }
public CustomList(List<T> list) : base(list)
{ }
public CustomList(CustomList<T> list) : base(list)
{ }
}
C) Finally subscribe to your event:
var myList = new CustomList<YourObject>();
myList.ListCountChanged += (obj, e) =>
{
// get the count thanks to e.NewCount
};

Is there a way I can add code to update a timer list into a class I inherit from?

I have the following classes:
public abstract class BaseGridViewModel
{
protected BaseGridViewModel()
{
Timer = new List<long>();
}
public List<long> Timer { get; set; }
}
public class CityReportViewModel : BaseGridViewModel
{
public IEnumerable<City.Grid> Grid { get; set; }
}
In my action code I am doing timing like this:
var vm = new CityReportViewModel();
var sw = Stopwatch.StartNew();
try {
//
vm.Timer.Add(sw.ElapsedMilliseconds);
//
vm.Timer.Add(sw.ElapsedMilliseconds);
//
// Lots more vm.Timer.Add lines ...
} catch (Exception e) {
log(e);
} finally {
sw.Stop();
}
Because I do a lot of timing I have the vm.Timer.Add code repeated many times in my controllers.
Is there some way I could simplify the timer coding by making a change to the ViewModels. The kind of thing I am thinking of is to set up the timer as a field in the ViewModels and then have some way of telling the ViewModel that I want to record a new timing event.

I'm not really sure about your requirement, but it sounds like you need an observer.
Your 'BaseGridViewModel' may register himself to an obeserver. The observer raises may an event to update all timers in all attached BaseGridViewModels.
Edit
This is what I'm talking about:
public class TimerUpdateService
{
public TimerUpdateService()
{
// May create here a Stopwatch to measure your time.
}
private EventHandler<EventArgs> _updater;
public void Register(BaseGridViewModel baseGridViewModel)
{
_updater += baseGridViewModel.timerUpdate_UpDateTimers;
}
public void Unregister(BaseGridViewModel baseGridViewModel)
{
_updater -= baseGridViewModel.timerUpdate_UpDateTimers;
}
/// <summary>
/// Call this method to refresh all timers on the registred 'BaseGridViewModel'.
/// </summary>
public void UpdateAllViewModels()
{
EventHandler<EventArgs> handler = _updater;
if (handler != null)
handler(this, EventArgs.Empty);
}
}
public abstract class BaseGridViewModel
{
protected BaseGridViewModel()
{
Timer = new List<long>();
}
public void timerUpdate_UpDateTimers(object sender, EventArgs e)
{
Timer.Add(e.elapsed);
}
public List<long> Timer { get; set; }
}
public class CityReportViewModel : BaseGridViewModel
{
public IEnumerable<City.Grid> Grid { get; set; }
}
May this is what you need. The above code is only an example..

Events + Adapter Pattern

I have an adapter pattern on a generic class that essentially adapts between types:
class A<T> { event EventHandler e; }
class Aadapter<T1, T2> : A<T1> { A<T2> a; Aadapter(A<T2> _a) { a = _a; } }
The problem is that A contains an event. I effectively want all event handlers assigned to Adapter to fall through to a.
It would be awesome if I could assign the a's event handler to adapter's event handler but this is impossible?
The idea here is that A is almost really just A but we need a way to adapt the them. Because of the way event's work I can't how to efficiently do it except manually add two event handlers and when they are called they "relay" the to the other event. This isn't pretty though and it would seem much nicer if I could have something like
class A<T> { event EventHandler e; }
class Aadapter<T1, T2> : A<T1> { event *e; A<T2> a; Aadapter(A<T2> _a) { a = _a; e = a.e; } }
in a sense we have a pointer to the event that we can assign a2's event to.
I doubt there is any simple way but maybe someone has some idea to make it work.
(BTW, I realize this is possible with virtual events but I'd like to avoid this if at all possible)

I think that this is what you are after:
class A<T>
{
public virtual event EventHandler e;
}
class Aadapter<T1, T2> : A<T1>
{
A<T2> a;
Aadapter(A<T2> _a) { a = _a; }
public override event EventHandler e
{
add { a.e += value; }
remove { a.e -= value; }
}
}
Or chain it
class A<T>
{
public event EventHandler e;
protected void ChainEvent(object sender, EventArgs eventArgs)
{
e(sender, eventArgs);
}
}
class Aadapter<T1, T2> : A<T1>
{
A<T2> a;
Aadapter(A<T2> _a)
{
a = _a;
a.e += ChainEvent;
}
}

Why is subscribing and forwarding events not pretty? I find it elegant.
Doing this is consistent with the way the rest of the adapter is implemented.
Even if you could use a pointer, it would be inconsistent because you wouldn't want to do that in every case.
For example, if you're adapting a class that implements INotifyPropertyChanged to an interface that does not, but exposes a couple of properties, such as "TitleChanged" and "MaxLengthChanged," you wouldn't then use a pointer. Your adapter would expose those two events, and consumers would subscribe. Your adapter would subscribe to the PropertyChanged event, and raise "TitleChanged" only when it gets notified that "Title" was modified, and "MaxLengthChanged" only when it gets notified that "MaxLength" was modified. All other notifications would be ignored.
I favour this approach as I find it straightforward, consistent and true to the pattern.

Example showing the "standard" methods to solve the problem. The first uses virtual events/methods while the second does a "doubled ended" forwarding scheme. Both have their pro's and cons but would be nice if there was an easier method that didn't grow with the number of events. What we would like to do is sort of combine the two events into one directly instead of indirectly which is what all this code does. (pointers would be such a method if they were possible in C#)
//#define __virtual
#define __direct
using System;
using System.Collections.Generic;
using System.Text;
namespace VirtualEvents
{
#if __virtual
#region
public class A<T>
{
public virtual event EventHandler e;
public virtual void Fire() { e(this, null); }
}
public class Aadapter<T1, T2> : A<T1>
{
A<T2> a;
public override event EventHandler e
{
add { a.e += new EventHandler(value); }
remove { a.e -= new EventHandler(value); }
}
public override void Fire()
{
a.Fire();
}
public Aadapter(A<T2> _a)
{
a = _a;
}
}
#endregion
#elif __direct
#region
public delegate EventHandler EventHandlerPtr();
public class eventPtr
{
public EventHandler _event;
}
public class A<T>
{
//internal EventHandler _event;
public eventPtr _event = new eventPtr();
public event EventHandler e
{
add { _event._event += value; }
remove { _event._event -= value; }
}
public void Fire() { _event._event(this, null); }
}
public class Aadapter<T1, T2> : A<T1>
{
A<T2> a;
public Aadapter(A<T2> _a)
{
a = _a;
this._event = a._event;
}
}
#endregion
#else
#region
public class A<T>
{
public event EventHandler e;
public void Fire() { e(this, null); }
}
public class Aadapter<T1, T2> : A<T1>
{
A<T2> a;
public Aadapter(A<T2> _a)
{
a = _a;
a.e += new EventHandler(a_e);
e += new EventHandler(Aadapter_e);
}
void Aadapter_e(object sender, EventArgs e)
{
a.e -= new EventHandler(a_e);
a.Fire();
a.e += new EventHandler(a_e);
}
void a_e(object sender, EventArgs e)
{
this.e -= new EventHandler(Aadapter_e);
Fire();
this.e += new EventHandler(Aadapter_e);
}
}
#endregion
#endif
class Program
{
static void Main(string[] args)
{
var a = new A<double>();
var q = new Aadapter<int, double>(a);
a.e += new EventHandler(a_e);
q.e += new EventHandler(q_e);
a.Fire();
q.Fire();
((A<int>)q).Fire();
Console.ReadKey();
}
static void a_e(object sender, EventArgs e)
{
Console.WriteLine("From a");
}
static void q_e(object sender, EventArgs e)
{
Console.WriteLine("From q");
}
}
}
(edit: the code now includes a new method which wraps the event in a class which now allows events to be assigned easily and effectively represents the "the pointer" case. Hopefully someone can improve upon these even more.)